The present invention relates to a method for bending a glass sheet, said method comprising the following steps:
a glass sheet is heated in a bending furnace close to its softening temperature, PA1 a heated glass sheet is supported on a ring mould, and PA1 a glass sheet is allowed to bend by gravity. PA1 a bending furnace fitted with heating devices for maintaining the furnace temperature sufficiently high for bending a glass sheet and PA1 a ring mould for carrying a glass sheet to be bent in the bending furnace.
The invention relates also to an apparatus for bending a glass sheet, said apparatus including:
The invention can be applied in association with various types of furnace plants. In one main type, a glass sheet supported on a ring mould is carried from on furnace section into another by increasing temperature in successive furnace sections. In the final section, a sufficiently high temperature is reached for bending. In another main type, a glass sheet is heated in a furnace fitted with rollers and is then lifted from the rollers e.g. by means of an overhead vacuum pick-up and lowered on top of a ring mould brought thereunder, whereafter the bending can be effected upon the ring mould in the furnace or in a separate bending section downstream of the furnace.
Regardless of a type furnace employed, a problem with bending effected on ring moulds has been the control of temperature and bending in various sections of a glass sheet. If the overall temperature of a glass sheet is increased to a sufficient level in order to effect the bending of difficult-to-bend margianl sections, the central section of a glass sheet is also softened to the extent that it bends and sags too far downwards.
It is prior known to overcome this problem by applying a more powerful heating effect to the marginal sections of a glass sheet for the sufficient bending thereof, but the central area of a glass sheet is left to be substantially colder (e.g. 580.degree. C.) for avoiding its excessive bending and sagging downwards. However, a resulting drawback in this method is that it is not possible to produce properly tempered glass since for a proper tempering effect the entire area of a glass sheet should be heated to over 600.degree. C., preferably to the temperature of appr. 610.degree.-620.degree. C.
An object of the invention is to provide a method and apparatus for bending a glass sheet on a ring mould substantially gravitationally while simultaneously heating the entire area of a glass sheet to a sufficient temperature for bending and, if necessary, also for tempering.
A particular object of the invention is to provide a method and apparatus for controlling and regulating the gravitational bending of the entire area of a glass sheet in a manner independent of the normal interrelation between gravity and temperature. An additional feature of the invention is a possibility of focusing the heating effect on the areas critical in terms of bending.
These and other objects of the invention to be described in more detail hereinafter can be achieved on the basis of the characterizing features set forth in the annexed claims.